1. Field of the Invention
The present invention relates to a display-integrated type tablet device concurrently having an image display function for use in a laptop computer, a word processor, or the like, and a method for driving the same.
2. Description of the Prior Art
Conventionally, there has been a display-integrated type tablet device formed by integratingly laminating an image display unit with a tablet. FIG. 16 shows a schematic view of an electrostatic induction tablet and a drive section thereof for use in such a display-integrated type tablet device.
An electrostatic induction tablet 21 is formed by placing a glass substrate on which column electrodes X.sub.1, X.sub.2, . . . , X.sub.m (an arbitrary column electrode referred to as X hereinafter) are arranged in parallel opposite to a glass substrate on which row electrodes Y.sub.1, Y.sub.2, . . . Y.sub.n (an arbitrary row electrode referred to as Y hereinafter) are arranged in parallel so that the electrodes cross each other at right angles and fixing the substrates with interposition of a spacer (transparent adhesive or the like). Each row electrode Y is connected to a row electrode shift register 22, while each column electrode X is connected to a column electrode shift register 23.
In the above case, the column electrode X and the row electrode Y are formed of indium tin oxide (ITO) or the like to be approximately transparent.
The row electrode shift register 22 and the column electrode shift register 23 are connected to a timing generation circuit 24. The timing generation circuit 24 is connected to an x-coordinate detection circuit 27 and a y-coordinate detection circuit 28. The x-coordinate detection circuit 27 detects the x-coordinate at the tip end of an electronic pen 25 based on a signal from the timing generation circuit 24 and a signal input from the electronic pen 25 via an operational amplifier 26, and outputs an x-coordinate signal representing the x-coordinate. In the same manner as above, the y-coordinate detection circuit 28 outputs a y-coordinate signal representing the y-coordinate at the tip end of the electronic pen 25.
The electrostatic induction tablet 21 having the above-mentioned construction has a light transmittance of approximately 85%. Therefore, when the electrostatic induction tablet 21 is laminated on a liquid crystal display, an image displayed on the liquid crystal display can be viewed through the electrostatic induction tablet 21. Therefore, the electrostatic induction tablet 21 is laminated on the liquid crystal display to construct a display-integrated type tablet device to allow coordinates on the liquid crystal display to be input by means of the electrostatic induction tablet 21 and the electronic pen 25.
The electrostatic induction tablet 21 and the drive section thereof operate in a manner as follows.
In detail, firstly shift data and a clock signal are transmitted from the timing generation circuit 24 to the column electrode shift register 23. Then scanning pulses of column electrode scanning signals x.sub.1 through x.sub.m as shown in FIGS. 17A and 17B are sequentially applied from the column electrode shift register 23 to each column electrode X. Then, in the same manner as above, scanning pulses of row electrode scanning signals y.sub.1 through y.sub.n are sequentially applied from the row electrode shift register 22 to each row electrode Y. In the above case, the electronic pen 25 is made to approach the surface of the electrostatic induction tablet 21.
With the above-mentioned operation, since the tip electrode (not shown) of the electronic pen 25 is coupled with each of the column electrode X and the row electrode Y through a stray capacitance, an induction voltage as shown in FIG. 18 (a) is induced at the tip electrode of the electronic pen 25. In the present case, the tip electrode is connected to the operational amplifier 26 to set up the impedance measured from the input side of the tip electrode higher than the impedance measured from the output side of the operational amplifier 26.
Thus based on the induction voltage induced at the tip electrode, the coordinates at the tip end of the electronic pen 25 are detected in a manner as follows.
In detail, an induction voltage signal having a waveform as shown in FIG. 18 (a) output from the electronic pen 25 is made through a low-pass filter and an amplifier to have a waveform as shown in FIG. 18 (b), and then input to the x-coordinate detection circuit 27 or the y-coordinate detection circuit 28.
The x-coordinate detection circuit 27 measures a time (Ts in FIG. 18 (b)) from the time when the scanning pulses of the column electrode scanning signal x.sub.1 as shown in FIG. 17 starts to be applied from the column electrode shift register 23 to the column electrode X.sub.1 to start scanning of the column electrode X to the time when a peak of the signal from the electronic pen 25 is input based on the clock signal from the timing generation circuit 24 and at the tip end of the electronic pen 25 on the electrostatic induction tablet 21. In the display-integrated type tablet device, by displaying on the display screen of the liquid crystal display a letter or a figure which has been input by tracing the surface of the electrostatic induction tablet 21 with the tip end of the electronic pen 25, the letter or figure can be input as if it were written on a paper with writing implements such as a ball-point pen.
However, the above-mentioned display-integrated type tablet device has the following problems.
First, when the surface of the electrostatic induction tablet 21 is traced by the electronic pen 25 while monitoring the display screen of the liquid crystal display, it is very hard to view the display screen of the liquid crystal display.
In detail, the column electrode X and the row electrode Y of the electrostatic induction tablet 21 are formed of tin oxide, indium oxide, or the like to have a transparency on a transparent substrate made of glass, plastic, or the like. However, the thus formed electrode has a light transmittance of approximately 85% being not so high in comparison with the light transmittance of the substrate, and is accompanied by a blur. The electrodes are arranged regularly in a grating form. Therefore, the the signal from the electronic pen 25. Then based on the measured value, an x-coordinate signal representing the x-coordinate at the tip end of the electronic pen 25 is output.
In the same manner as above, the y-coordinate detection circuit 28 measures a time from the time when scanning of the row electrode Y starts to the time when a peak of the signal from the electronic pen 25 is input. Then based on the measured value, a y-coordinate signal representing the y-coordinate at the tip end of the electronic pen 25 is output.
In the above case, the time Ts is measured by counting the number of pulses of the clock signal applied to the row electrode shift register 22 or the column electrode shift register 23.
As described above, since the electrostatic induction tablet 21 can detect the coordinates at the tip end of the electronic pen 25 with high accuracy regardless of its relatively simple construction, the tablet 21 is extensively used in a small computer or the like.
An electrostatic induction tablet 21 as described above and a liquid crystal display (not shown) are laminated to constitute a display-integrated type tablet device in which image display is effected on the pixel of the liquid crystal display corresponding to the coordinates electrodes X.sub.1, X.sub.2, . . . , X.sub.m, Y.sub.1, Y.sub.2, . . . Y.sub.n of the electrostatic induction tablet 21 are conspicuous more than expected. The above-mentioned phenomenon is significant particularly in a simple type display-integrated type tablet device provided with no back light.
Furthermore, a comparatively great area of the display screen of the liquid crystal display is covered with the electrodes X.sub.1, X.sub.2, . . . X.sub.m, Y.sub.1, Y.sub.2, . . . Y.sub.n of the electrostatic induction tablet 21. As a result, the display screen of the liquid crystal display is dark and has a low contrast.
Furthermore, since the liquid crystal display and the electrostatic induction tablet 21 are constructed separately, it is possible that the liquid crystal display and the electrostatic induction tablet 21 are relatively displaced when the liquid crystal display and the electrostatic induction tablet 21 are laminatedly assembled to be integrated. When such displacement takes place, there is disadvantageously generated a displacement between the pen-input position on the liquid crystal display (the position pointed by the tip end of the electronic pen 25) and the position of the pixel on which image display is effected according to the pen input on the display screen of the liquid crystal display. The above fact hinders any letter or figure from being input as if it were written on a paper with writing implements such as a ball-point pen.
Furthermore, since the liquid crystal display and the electrostatic induction tablet 21 prepared separately are laminatedly assembled to be integrated, the resulting display-integrated type tablet has a great thickness and heavy weight. The above fact results in a serious hindrance in achieving compacting of a small-size computer or a word processor demanded by consumers and a cause for cost increase.
Taking the aforementioned facts into consideration, there is proposed a display-integrated type tablet device as follows.
A first display device with tablet function has an electronic pen electrostatically coupled with the row electrode and the column electrode of an active matrix liquid crystal display (LCD) panel. A period in which the active matrix LCD panel is driven is divided into an image display period and a coordinate detection period. In the coordinate detection period, scanning pulses having a voltage which exerts no influence on image display are successively applied to the row electrode and the column electrode to scan both the electrodes. In the scanning stage, a voltage induced at the electronic pen is detected to detect the coordinates at the tip end of the electronic pen based on the scanning timing of both the electrodes.
A second display device with tablet function has an active matrix LCD panel in which row electrodes and column electrodes for image display and row electrodes and column electrodes for coordinate detection are arranged, an image display pixel electrode connected to both the electrodes via a switching element is provided in each position where the row electrode and the column electrode for image display cross each other, and a coordinate detection pixel electrode connected to both the electrodes for coordinate detection via a switching element is provided in each position where the pixel electrode and the row electrode for coordinate detection cross each other. The electronic pen is coupled with the pixel electrode for coordinate detection in the active matrix LCD panel through an electrostatic capacitance.
The display device with tablet function scans the column electrode and the row electrode for coordinate detection in the active matrix LCD panel to charge the pixel electrode for coordinate detection with electronic charges, and detects the electronic charges by means of the electronic pen to detect the coordinates at the tip end of the electronic pen based on the scanning timing of both the electrodes.
However, each of the above-mentioned display devices with tablet function has the following problems.
In the first display device with tablet function, one frame period is divided into the image display period and the coordinate detection period and the coordinates at the tip end of the electronic pen is detected in the coordinate detection period. Therefore, it is impossible to provide a long coordinate detection period for the purpose of assuring an image display period having a specified duration when the display panel has a great area to have an increased number of row electrodes and column electrodes. The above fact results in the problem that the electrode scanning speed is inevitably increased to consequently reduce the accuracy in detecting the coordinates at the tip end of the electronic pen.
Furthermore, in the coordinate detection period, it is required to scan in the coordinate detection period with scanning pulses having a polarity reverse to that of the scanning pulses successively applied to the row electrodes in the image display period so as to prevent each TFT (Thin Film Transistor) i.e. a switching element from being turned on to consequently change the voltage applied to the liquid crystals. Therefore, it is required to incorporate the function of generating in the coordinate detection period the scanning pulses having a polarity reverse to that of the scanning pulses used in the image display period into the row electrode scanning circuit of the normal active matrix type LCD panel, which causes the problem that a complicated row electrode scanning circuit results.
In the second display device with tablet function, it is required to provide the active matrix type LCD panel with the column electrode and the pixel electrode for image display as well as the column electrode and the pixel electrode for coordinate detection. The above fact results in a complicated active matrix LCD panel construction to consequently cause the problems of reduction in productivity and cost increase.
Furthermore, among the pixel electrodes in the active matrix LCD panel, the pixel electrode which is practically used for image display is only the image display pixel electrode. Therefore, the panel numerical aperture (a ratio of the area of image display pixel electrodes that practically contribute to image display to the area of the display screen of the panel) reduces. The above fact also results in the problem that the image display quality reduces.